Process for concentrating acetic acid



Jan. 28, 1936. OTHMER 2,028,801

PROCESS FOR CONGENTRATING ACETIC ACID Original Filed Feb. 6, 1931 2 Sheet'sSheet l IFJIIILL mm.

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INVENTOR. Donald E Othmer Jan. 28, 1936.

. OTHMER Original Filed Feb. 6, 1931 2 Sheets-Sheet 2 Fllfllfi.

v r A V I 44 69 my 54 n 55 56 73 44 4Z 4- T INVENTOR. 7 Donald Othmer BY I $MT J7? ATTORIEEYS Patented Jan. 28, 1936 UNITED STATES PATENT, OFFICE PROCESS FOR CONCENTRATING ACETIC ACID Donald F. Othmer, Brooklyn, N. Y.

16 Claims.

This invention relates to an improved process for the concentration of acetic acid from aqueous solutions thereof, and more particularly to processes employing halogen containing compounds as withdrawing agents. It is applicable to solutions of practically any percentage or strength, as well'as to those which may be contaminated by impurities in solution or suspension. The process of this invention is furthermore applicable to solutions of acid irrespective of how such solutions may have been formed or obtained, as for example, solutions involving pyroligneous acid, acetic acid obtained as a waste product in acetylation processes such as the acetylation of cellulose, fermentation acid, etc.

This application is a division of my application Serial No. 513,989, filed February 6, 1931.

Among the numerous methods known for the concentration of acetic acid solutions, two may be mentioned. One is the method of extracting the acid from its aqueous solution with some material which is a. solvent for the acid, and which is relatively insoluble in water. Another is the method of removing the water or dehydrating the acid by means of a distillation operation employing some'added material which may be called a withdrawing agent, which forms with water on heating, a constantboiling or azeotropic mixture having a boiling point sufiiciently lower than that of the acetic acid to permit the rectification of the mixture and the resulting removal of water from the acetic acid solution in the usual operations and equipment familiar in the art of distillation.

It is primarily with the second or distillation method that the present invention is concerned, although certain aspects of the invention also make use of the features of the first or solvent method. 7

One object of the invention is to provide a generally improved, more eflicient, and more satisfactory process for concentrating or dehydrating acetic acid.

Still another object is the provision of a proces of a simple and effective nature capable of employing withdrawing agents from a class heretofore believed to be unsuitable.

Another and very important object is to provide an'improved process which results in the complete rectification of acetic acid without leaving any residue of the withdrawing agent therein.

A further object is the provision of a process which will save a substantial amount of the heat which it has heretofore been necessary to use in prior processes of concentrating acetic acid.

A still further object is the provision of a process employing a withdrawing agent which may possess the property of a solubility for acetic acid from its aqueous solutions so that the withdrawing agent may also be used as an extracting agent for extracting acetic acid from the solution.

Another object is the provision of an improved, more efiicient, and more satisfactory continuous process for the concentration of acetic acid.

To these and other ends the invention resides in certain improvements and combinations of parts, all as will be hereinafter more fully described, the novel features being pointed out in the claims at the end of the specification.

In the drawings:

Fig. 1 is a vapor composition curve of a mixture of acetic acid and water;

Fig. 2 is a vapor composition curve of a mixture of acetic acid and benzene;

Fig. 3 is a vapor pressure curve illustrating the vapor pressures of water, propylene chloride, and an azeotropic mixture of the two;

Fig. 4 is a vapor pressure curve illustrating water, benzene, and an azeotropic mixture of the two;

Fig. 5 is a diagrammatic view of a form of apparatus for carrying out the process.

Similar reference numerals throughout the several views indicate the same parts.

The general use of 'a withdrawing agent in concentrating or dehydrating an aqueous solution of acetic acid is explained and set forth in British Patent No. 327,444, accepted April 3, 1930, which patent is the result of a prior invention made jointly by Hans T. Clarke and the present applicant, Donald F. Othmer. Reference is made to said British patent for a disclosure of the general principles underlying the use of a withdrawing agent (referred to as an auxiliary liquid in said patent), which need not be repeated here. The said patent sets forth eight chief requirements for an auxiliary liquid or withdrawing agent for use in distilling water from aqueous acetic acid, which eight requirements still hold in connection with the present invention, with the exception of requirement No. 3. This states that the withdrawing agent should boil at a lower temperature than acetic acid so that it would be readily and completely separable from the latter by distillation.

It should be noted that the present invention contemplates the use of withdrawing agents which do not boil sufficiently lower than acetic acid to permit such complete separation readily by distillation.

A This fact is not found to be detrnnental because, according to the present invention, when the process is properly and etflciently carried out according to the preferred embodiment of this invention the withdrawing agent does not become mixed with the concentrated acetic acid.

The withdrawing agents which have heretofore been considered suitable for use in concentrating acetic acid may be divided in general into two classes. The first comprise what might be termed the low boiling point class, such as ethylene dichloride disclosed in the aforesaid British Patent No. 327,444, benzol disclosed in United States Patent No. 1,722,532 to mude, and ethyl acetate disclosed in British Patent No.

284,588 to I. G. Farbenindustrie Aktiengesellschaft. The second group comprises what might be termed the high boilingpoint class such as butyl acetate, as disclosed in British Patent No. 298,137 to Dr. Alexander Wacker Gesellschaft.

Withdrawing agents of the low boiling point grcup have been considered suitable because any excess of withdrawing agents could be readily separated from the concentrated acetic acid by distillation. Withdrawing agents of the high boiiing point group have been unsuitable from the standpoint of easy separation cf excesses by distillation, but it has been possible to use them in some circumstances by following the method disclosed in said British Patent No. 298,137. Withdrawing agents boiling between the boiling points of the low boiling group and the high boiling group have heretofore been considered unsuitable and impracticable for use because of the diiiiculty of separating excesses of such withdrawing agents from the concentrated acetic acid.

I have now discovered, however, according to the present invention that not only are withdrawing agents in an intermediate range between 88 C. and 103 C. suitable for use instead of being unsatisfactory as heretofore believed, but I have further discovered that such withdrawing agents are more suitable and better adapted for the purpose of concentrating acetic acid than many of the other withdrawing agents previously used.

The function of a withdrawing agent is to form with the water in the acetic acid solution a mixture which will boil at a constant hailing point below the boiling point of either water or the withdrawing agent alone. Such a constant boiling mixture is commonly known in the art as an azeotropic mixture and will be hereafter referred to by that designation.

Most azeotropic mixtures having water as one of the components, such as the mixtures hereinafter more fully discussed, are found to follow in general the known principles of steam distillation. It is also found that if water be mixed with a liquid which is substantially immiscible therewith, the respective vapor pressures of the water and the other liquid are each substantially unaflected by the presence of the other, and the vapor pressure of the mixture is very nearly equal to the sum of the individual vapor pressures of the two liquids. Hence a vapor pressure curve of the mixture may be drawn by summing the separate vapor pressure curves of the two component liquids.

Also it is found that the molecular compositlon of vapors arising from such mixtures is substantially proportional to the respective vapor pressures of the constituents. The higher the vapor pressure of the other liquid mixed with water, the greater will be the number of molecules of such other liquid any given quantity of vapor evolved from the mixture.

The emciency oi a concentration process involving an azeotropic mixture of water and a withdrawing agent depends to a. material extent on the relative proportions of water and withdrawing agent which are vaporized. This depends, in turn, upon the relative molecular percentages of the water and of the withdrawing agent, which molecularpercentages, as above stated, are approximately proportional to the respective vapor pressures of the water and the withdrawing agent at the particular pressure under which boiling takes place, generaliy and preferably atmospheric pressure, although subatmospheric or superatmospheric pressures may be used if desired. Since the normal boiling points of liquids are temperatures at which their vapor pressures are the same, i. e. 76 centimeters of mercury, it follows that the vapor pressures of different liquids vary in general more or less inversely with their boiling points. Therefore a statement of the boiling point oi a liquid gives a general rough indication of its relative vapor pressure characteristics, the liquids of higher boiling point having in general at all temperatures lower vapor pressures than liquids of lower boiling point.

Hence from the standpoint of. efliciency as determined by the vapor pressure of the withdrawing agent, it is desirable to use withdrawing agents having relatively high boiling points and consequently lcwer .vapor pressures. At the same time, the use of a withdrawing agent having a high boiling point is accompanied by the disadvantage that such an agent will not lower the boiling point of an azeotropic mixture with water to a suflicient extent to permit easy distillation of the azeotropic mixture from the acetic ing agents suitable for use, the advantages of higher or lower boiling points must be balanced with the corresponding disadvantages, and suitable agents must be selected which have to as large an extent as practicable, the advantages of high boiling point withcut an inipractically large amount of the disadvantages thereof.

I have discovered that some very suitable withdrawing agents for use in concentrating aqueous acetic acid solutions, are those which have boiling points at normal atmospheric pressure of more than 88 C. and less than 103 C. Such withdrawing agents which have boiling points between 88 C. and 103 C. have in general vapor pressures which, in azeotropic mixtures at or near normal atmospheric pressure, are not materially greater than the vapor pressure of the water in the azeotropic mixtures, so that a process emof the azeotropic mixture from the aceticv acid soltion and from pure acetic acid.

Viewed from a slightly different aspect, I find my withdrawing agents are those whose vapor pressures, at the temperature of the boiling point of the azeotropic mixture with water, are not more than 60% nor less than 40% of. the total vapor pressure of the azeotropic mixture. When the vapor pressure of the withdrawing agent is more than about 60% of the total vapor pressure of the azeotropic mixture, then the molecular percentage of the vapor from the azeotropic mix solution and from pure acetic acid. Hence the vapor pressure limits of 60% and above mentioned have been found to be very suitable, and

the present invention contemplates the use of withdrawing agents having a vapor pressure between these limits.

Viewing the matter from still another aspect, I find that in general suitable withdrawing agents are those which form azeotropic mixtures with water in which the azeotropic m xture has a normal boiling point between 76 C. and 86 C. The three sets of limits above specified correspond in general with each other. .That is..withdrawing agents whose vapor pressures are within the specifled limits of 60% and 40% will generally be found to have boiling points approximately between the specified ranges of 88 C. and 103 C., and it will further be found that such agents form azeotropic mixtures with water which mixtures have boiling points approximately between the specified limits of 76 C. and 86 C.

Thus it is the physical or physical chemical properties rather than the strictly chemical properties of the withdrawing agents which are found to be most important according to the present invention. Of course all liquids which have boiling points between 88 C. and 103 C. would not be suitable for the purposes of the present invention, because all of such liquids would not function as withdrawing agents. A withdrawing agent. as the term is understood in the art and as it is intended in this specification and in the accompanying caims. may be defined as a liquid which will form a constant boiling or azeotropic mixture with water. Generally and preferably. a withdrawing agent is substant ally immiscible with water, although some withdrawing agents which are miscible with Water are known. Those withdrawing agents which are substantially immiscible with water form azeotropic mixtures therewith in which the vapor pressure of the mixture at any temperature is approximately the sum of the separate vapor pressures of water and the withdrawing agent.

Thus, by definition, the term withdrawing agent as herein used automatically excludes al substances which will not form azeotropic mixtures with water. I do find, according to the present invention, that most any halogen containing withdrawing agent having a boiling point between 88 C. and 103 C. is suitable and satsfactory for use in the concentrating of acetic acid, whereas withdrawing agents boiling within this range of temperatures have heretofore been considered entirely unsuitable and impracticable.

As examples of suitable halogen containing withdrawing agents boiling within this temperature range at normal atmospheric pressure, I have found the following materials to be more or less advantageous for the process described, the approximate boiling point under normal atmospheric pressure being indicated after the name of each substance:

Degrees centigrade Iso-propyl iodide 89.5 Dichlorobrommethane 90.2 Iso-butyl bromide 91.6

'Dibrommethane 97 Iso-amyl chloride 99 Normal butyl bromide 101 Allyliodide 102 Certain of these substances, such as aliyl iodide form azeotropic mixtures at certain percentages with acetic acid as well as with water, and consequently have been found useful in concentrating acetic acid only between certain ranges of strength or in concentrating acetic acid where an anhydrous acid mixed with some proportion of withdrawing agent may be discharged for use in some further operation. The use of a withdrawing agent which forms an azeotropicmixture with acetic acid is not generally desirable, b'ut may be suitable under special circumstances as above mentioned. Benzene, known in the prior art as a withdrawing agent (see Maude Patent No. 1,722,532, above mentioned), forms an azeotropic mixture with acetic acid when the mixture contains about 98% benzene, but this does not prevent its use under some circumstances. Similarly the use of ;a withdrawing agent within the boiling range specified, such as allyl iodide, which forms an azeotropic mixture with acetic acid, makes it diflicult or uneconomical to concentrate dilute aqueous acetic acid by the preferred process described, but if the amount of water in the starting solution is only comparatively small, the acid may be discharged from the still pot in a practically pure condition although the water will also contain substantial amounts of acid.

1 Other withdrawing agents, which do not form azeotropic mixtures with acetic acid, are in general more suitable than those which do form such mixtures. It has been found, of the halogen containing compounds described, that exceedingly satisfactory results are obtained by using propylene chloride and at present it is preferred to use this substance which is commonly available at reasonable cost, the subject matter of Patent application C. N. 513,989 is directed in some extent to this particular agent.

To aid in understanding the physical properties of the withdrawing agents and mixtures of the present invention, various curves are illustrated in Figs. 1 to 4 inclusive. Referring to these, Fig. 1 shows what is called a vapor composition curve of water and acetic acid. The abscissa: of the graph are the percentages of water in the mixture, while the ordinates are the percentages of water vapor in the total vapor evolved by boiling. The line is a reference line for the sake of comparison and indicates a percentage of water in the vapor equal to the percentages of water'in the liquid mixture. It will be seen that the vapor composition curve of water and acetic acid diverges only to,a comparatively slight extent from the 45 line so that only slightly more water vapor than acetic acid vapor is given an when a mixture of the two is boiled. This indicates that a simple mixture or water and acetic acid cannot be satisfactorily concentrated by distillation, and shows why it is necessary to use a withdrawing agent in order to concentrate aqueous acetic acid economically.

Fig. 2 illustrates a similar vapor composition curve of benzene and acetic acid in which ,the percentages indicated refer to benzene. The main curve is plotted on the main abscissa and the main ordinates indicated at the bottom and left hand side of the graph, respectively, while the supplementary curve is an enlargement oi! the upper right hand end of the main curve and is plotted on the enlarged scale abscissa and ordinates indicated at the top and right hand sideagents, is limited.

Referring now to Fig. 3 there is shown a vapor pressure curve of water indicated by the line III, a vapor pressure curve of propylene chloride indicated by the line I I, and a vapor pressure curve of an azeotropic mixture of water and propylene chloride indicated by the line [2. The total vapor pressure of the mixture at any particular temperature is substantially the sum of the two separate vapor pressures at that temperature, as explained above. In this graph, the abscissa are graduated in degrees centigrade from '50 to 100, as the portions of the curves below 50 C. are unnecessary for the present purposes. The ordinates are graduated in pressures expressed as centimeters of mercury from 0 to 80. The reference line i3 indicates a pressure of 76 centimeters or normal atmospheric pressure. Similar graphs would be obtained from plotting data concerning the other halogen compounds described above.

It will be seen that the curve l2 of the mixture crosses the line l3 of normal atmospheric pressure at approximately 78 0., indicating this as the boiling point of the mixture, which is between the limits of 76 C. and 86 C. above mentioned. Of the total vapor pressure of 76 centimeters of mercury, about 33 centimeters is due to the vapor pressure of the water, as indicated by the bracket l4, while the remainder or about 43 centimeters is due to the vapor pressure of the propylene chloride, as indicated by either of the two brackets I5 which subtend an equal distance. Hence it will be seen that the vapor pressure of the propylene chloride at the boiling point of the mixture is about 57% of the total vapor pressure of the mixture, and is between the limits of 60% and 40% above mentioned.

As the vapor pressures of the components of a mixture of this kind are very nearly proportionate to the respective molecular percentages, it follows that of every '16 molecules in the vapor evolved from the liquid mixture, about 33 molecules will be water molecules and about 43 molecules will be propylene chloride molecules. By reducing to percentages and multiplying the molecular percentage by the molecular weights, the relative quantities by weight of water and of propylene chloride evolved from boiling and azeotropic mixture of the two can be readily calculated. When this is done, it is found that of every unit of weight of the mixture which is distilled, approximately 11% by weight is water and approximately 89% by weight is propylene 5 chloride. In) other words, for every unit by weight of water which is distilled from an azeotropic mixture of water and propylene chloride, it is necessary to distill eight units by weight of propylene chloride. Multiplying this by the 10 latent heat of vaporization of propylene chloride, which is 75 calories per gram, it is found that 600.. calories are necessary to distill the quantity of withdrawing agent needed to carry over each gram of water in the mixture. This is relatively l5 efllcient in comparison to withdrawing agents which have been used in the prior art, in which much greater proportions of withdrawing agents have ordinarily had to be distilled, with consequent greater consumption of heat. 20

For the sake of comparison, Fig. 4 shows a similar set of vapor pressure curves of water and benz ene, one of the prior art withdrawing agents, and of a mixture of the two. The curve 20 is that of water, 2| that of benzene, and 22 that of 25 the azeotropic mixture of the two. Line 23 indicates normal atmospheric pressure of '76 centimeters, and crosses the line 22 at about 68, indicating this as the boiling point of the mixture. The bracket 24 indicates that of the total vapor pressure of 76 centimeters only a relatively small proportion or about 22 centimeters is due to the vapor pressure of the water, while a relatively large proportion or about 54 centimeters is due to the vapor pressure of the benzene as indicated by the brackets 25. Hence when using benzene, according to the prior art, it is necessary to distill many more molecules of the withdrawing agent for each given quantity of water molecules distilled than when using propylene chloride according to the present invention. It will be seen that the vapor pressure and consequently the molecular percentage of the benzene is more than twice that of the water, and about 71% of the total vapor pressure of the mixture. When multiplying the molecular percentages by the molecular weights as above indicated, it is found that for every unit of water distilled from an azeotropic mixture of water and benzene, it is necessary to distill 10.6 units by weight of benzene, and since the latent heat of vaporization of benzene is 94, it requires almost exactly 1000 calories to distill the withdrawing agent necessary to carry over one gram of water from the mixture. r This will be seen to be much more wasteful of heat than when propylene chloride is used according to the present invention.

The above comparisons show the halogen containing compounds, and in particular propylene chloride, to be a great deal more efficient than when benzene is used as the withdrawing agent. While I have referred to propylene chloride in particular, I also contemplate using any other halogen containing withdrawing agent boiling 65 within the temperature range of 88 C. to 103 C., as above mentioned.

The boiling points, molecular percentages, and other figures given above by way of example are those obtained by calculation. In actual prac- "0 tice the results are found to be slightly different from but quite close to those calculated, and in general it is found that the actual results are to a slight extent even more advantageous and eflicient than the calculated ones.

The various temperatures and vapor pressures above mentioned have been determined on the assumption that distillation will take place at normal atmospheric pressure of 76 centimeters of mercury, since this is ordinarily preferred. It is possible, however, to carry out the process of the present invention at pressures either above or below normal atmospheric.

Referring now to Fig. 5 of the drawings, there is shown a diagrammatic representation of an embodiment of apparatus for carrying out the present invention. The numeral 40 indicates a column still of any suitable and well known construction. For example, the still may be of the type having a multiplicity of plates and the usual bubble caps. It is heated in any suitable manner, such as by the coil 4| to which steam or other heating fluid is supplied from the inlet 42 and discharged from the outlet 43.

To aid in controlling the process, the still is provided at intervals throughout its height with suitable temperature indicating devices such as the thermometers 44.

To start the process the still is initially charged with the aqueous acetic acid which is to be concentrated, and with the withdrawing agent selected for use, supplied through the conduit 45 from the container 46. Ordinarily, after the still is initially charged with withdrawing agent, the valve in the conduit 45 is closed and no further withdrawing agent is introduced from the container 48 except as small amounts may be required from time to time to make up for losses in the system by leakage. The withdrawing agent with which the still is originally charged flows through a continuous circuit and is returned to the still for use over and over again, as described hereinafter.

In-the manner above described, the withdrawing agent will form an azeotropic mixture with the water in the aqueous acetic acid and the boiling point of this azeotropic mixture will'be sufliciently lower than the boiling point of acetic acid so that the azeotropic mixture will be vaporized and will pass off from the top of the still through the conduit 50 and into the condenser 5| where it is condensed and whence it issues from the outlet 52 and flows to the gravity or other suitable separator 53. The acetic acid, meanwhile, gradually works downwardly in the column and may be removed from the bottom thereof in liquid phase if desired, although it is preferred to remove the concentrated acetic acid in vapor phase through the outlet 54 leading to the condenser 55, where it is condensed to liquid phase and discharged through the conduit 56 to any desired point.

The process is preferably carried on continuously once the still has been charged and started,

additional aqueous acetic acid being supplied to the still through the inlet conduit 60 leading from any suitable source of supply such as the reservoir 6|. The aqueous acetic acid supplied through the conduit 60 may be in either liquid or vapor phase, and the conduit may enter the column at an intermediate point in the height thereof, which point will be determined in the usual manner well understood by those skilled in the art.

The azeotropic mixture of water and withdrawing agent, after entering the separator 53, separates into two layers, a lighter layer at the top and a heavier layer at the bottom. A conduit 64 leads from the layer of withdrawing agent to the still so that the withdrawing agent after being separated is returned substantially continuously to the still to be used over and over again. In case the withdrawing agent employed is lighter than water, the top layer 65 in the separator 53' will be agent while the bottom layer 66 will be water. Consequently the conduit 64 would be connected to the separator 53 near the top thereof, and a water discharge conduit 61 leads from a point near the bottom of the separator. In case the withdrawing agent employed is one which is heavier than water, however, the relative positions of the layers in the separator will be reversed, of course, and the relative positions of the conduits 64 and 61 will be correspondingly reversed.

While the separator 53 substantially separates the withdrawing agent from the water, there may be some slight amount of withdrawing agent dissolved in the water. Frequently the amount of withdrawing agent dissolved in the water is not suiiicient to justify recovery thereof, and the conduit 61 may then lead to waste, but if it is desired under any particular economic conditions to recover the withdrawing agent contained in the watery layer then the conduit 61 may be led to any suitable recovery apparatus, such for example as the auxiliary still 68 supplied with steam through the inlet 69. In this still, the withdrawing agent is separated from the water and the vapors of the withdrawing agent pass off from the top of the still through the conduit 10 to a condenser II in which they are condensed and from which they are led through the conduit 12 to the separator 53. Thus any withdrawing agent contained in the water layer of the separator is recovered and returned to the separator, whence it, together with the withdrawing agent separated from the'water in the separator, is supplied to the main still 40 for further use. The water separated from the withdrawing agent in the auxiliary still 68 may be discharged therefrom through the discharge conduit 13.

Since substantially all of the withdrawing agent distilled off from the column still throughthe conduit 50 returns to the column through the conduit 64, it follows that the rate of supply of the withdrawing agent to the column is ordinarily not variable, when the proper amount is in the system. The variable factors of the process are the rate at which heat is supplied through the conduit 42 and the rate at which acetic acid is supplied through the conduit 60. These two rates are varied as desired in order to obtain proper operation of the apparatus.

It is ordinarily desirable in operation to have at all points in the column the correct azeotropic ratio of withdrawing agent to the water in the acetic acid, even though it is only at the top of the column that the vapors are composed wholly of the azeotropic mixture to the exclusion of acetic acid vapor. At successively lower points in the column, where acetic acid is present in relatively greater and greater amounts and water is present in relatively smaller and smaller amounts, it is nevertheless desirable to maintain the theoretically correct azeotropic ratio between the water and the withdrawing agent. Such maintenance of correct ratios throughout the column constitutes the ideal case, and in practice it is ordinarily not possible to maintain this ratio correct at every point, but it may be maintained in a general or approximate Way. For example, a branch of the conduit 64 may lead t some lower point of the column at which a small fraction of the returning withdrawing agent may be supplied.

Heretofore, prior processes of concentrating acetic, acid have usually employed excesses of withdrawing agent, rather than jusfithe right amount of withdrawing agent as contemplated by the present invention. Hence withdrawing agent has heretofore been present at most W ts of the column, the lower portion of the column having no water present but only a mixture of acetic acid and withdrawing agent. 7

The presentinvention contemplates and it is of great importance, that the variable factors. (rate of heating and rate of supply of aqueous acetic acidl should be so controlled that there will be no excess of withdrajwing agent, and that at the point in the column at which the last of the water disappears, the last or the withdrawing agent will also disappear.

By watching the thermometers or other heat indicating devices 44 at the various points up and down the coiumn, an operator skilled in the operation of a column still is enabled to ascertain very closely the composition of the mixture at various points in the column, and in this way he is enabled to control the process to accomplish the desirable; result above set forth, varyingfthe rate of heating and acetic acid supply as necessary; and adding additional withdrawing agent from time to time if this is found to be necessary. When the process is carried on in the: above desired manner according to the present izfifention, the still pot will contain substantially pure or glacial acetic acid, instead of acid having withdrawing agelnt mixed therewith as has usually been the case heretofore. Consequently there will be no problem of separating theL withdrawing agent from the dehydrated acetic acid, and the latter will flow fromconduit 56 in substantially pure form ready for any desired use.

When the withdrawing agent employed in the process is one of those falling within the scope of the invention, which are more efficient in general than the prior art githdrawing agent for the reasons previously mentioned, it is found that the volume of vapors distilled frpm the column to obtain a given amount of glacial acetic acid is substantially less thani would be the case if less eilicient withdrawingjagents'such asthose of the prior artwere employed. Hence the various parts of the apparatus can be made smaller than would otherwise be the case, since a less quantity of material need be handled, and this results in asubstant ial saving in cost as apparatus of this characteris generally expensive because of its necessary lchemical resisting qualities. i

Accord ng to a modified process contemplated the present invention, instead of adding a withdrawing agent to the dilute acetic acid, the

withdrawing agent may be formed by chemical reaction in the dilute acetic acid itself, the substance added reacting either with e acetic acid or with the water which dilutes Hence the present invention is intended t ncludle withdrawing agents iormed or; added this manner, provided they fall within the temperature and other ranges specified, as well as those formed externally and added later to the dilute acetic acid. e

The present invention also contemplates the use as a withdrawing agent of a mixture of any two or more substances which when mixed will boil or have other characteristics within the ranges heretofore specified, regardless of what the boiling points or other characteristics of the individual components before mixture may be. '5 According to another embodiment of the process, instead of passing the withdrawing agent;

directly from the separator '53 to the still column 40, through the conduit 64', some fraction of it may first be led to an extractor of any suitable known type for extracting one liquid by another, in which extractor the withdrawing agent will come into contact with aqueous acetic acid and extract the acetic acid from the water. The mixture of withdrawing agent; acetic acid, and any small amount of water which is unavoidably present; is then supplied to the column still and rectified in the manner previously described, to remove whatever watl cr is present. r

The bulk of the water which was originally mixed witli the acetic acid is discharged from the extractor saturated with the withdrawing agent, and it may then be introduced through the conduit '61 into the auxiliary still 68 to have the withdrawing agent recovered in the manner previously described, the water free from withdrawirig agent being discharged through the conduit 13.

fWhile certain embodiments of the invention have been disclosed, it is to be understood that tlje inventive idea may be carried out in afirumber of ways. This application istherefore not to be limited to the precise details described, but is intended to cover all variations and modifications thereof falling within the spirit of the lnvention or the scope of the appended claims.

What I claim is? I 1. In the process of dehydrating aqueous acetic acid, the step of supplying aqueous acetic acid to a' distilling column, during distillation therein adding at least one halogen containing compound from the group consisting of iso-propyl iodide, dichlorobrommethane, iso-butyl bromide, dibrommethane, iso-amyl chloride, normal-butyl bromide, and allyl iodide at the upper part of the column and removing from the top of the column a constant boiling mixture of water and halogengcompoun'd.

2. In the process of dehydrating aqueous acetic acid, the step of mixing at least one halogen containing gompound from the group consisting of :iso-pronyl iodide, dichlorobrommethane, iso- Tnutyl bromide, dibrommethane, iso-amyl chloiride, normal-butyl bromide and allyl iodide with theaqueous acetic acid, distilling water and halo- ,gen compound from the mixture, condensing the distillate, allowing it to settle into two layers, ;and returning the layer'predorninating in halo- 'gen containing agent to the original mixture undergoing distillation. Z.

3. In the process of removin ater from aque ous acetic acid the steps of mix ng therewith at least one halogen containing compound from the group consisting of isof propyl iodide, dichlorobrommcthane, iso-butyl bromide, dibrommethane, iso-amyl chloride; normal-butyl bromide, and allyl iodide and distilling water and halogen compound from the mixture at a temperature below the boilinggnoint oflfwater.

4. In the process of dehydrating aqueous acetic acid, the step ;of supplying aqueous acetic acid containing vapors to a distilling column, during distillation therein adding at least one halogen containing compound from the group consisting of iso propyl iodide, dichlorobrommethane, iscbutyl bromide, dibrommethane, iso-amyl chloride, normed-butyl bromide, and allyl iodide at the up; per half of the column and removing from the top of thecolumn a constant boiling mixture of water and halogen containing compound.

5. In the process of dehydrating aqueous acetic acid the step of mixing iso-amyl chloride with the aqueous acetic acid and distilling water and isoamyl chloride from the mixture. I

6. In the process of dehydrating aqueous acetic acid, the step of mixing iso-amyl chloride with the aqueous acetic acid, distilling water and isoamyl chloride from the mixture, condensing the distillate, allowing it to settle into two layers, and returning the predominating layer in iso-amyl chloride to the original mixture undergoing distillation.

'7. The process of concentrating aqueous acetic acid which comprises mixing therewith not more than the right amount of at least one water withdrawing agent selected from the group consisting of iso-propyl iodide, dichlorobrommethane, isobutyl bromide, dibrommethane, iso-amyl chloride, normal-butyl bromide, and allyl iodide, to form an azeotropic mixture with the water in the aqueous acetic acid and distilling water and withdrawing agent from the mixture.

8. The continuous process of concentrating aqueous acetic acid to produce a concentrated acid free of withdrawing agent which comprises charging a still with a mixture of aqueous acetic acid and just the right amount of a water-withdrawing agent, selected from the group consisting of iso-propyl iodide, dichlorobrommethane, iso-butyl bromide, dibrommethane, iso-amyl chloride, normal-butyl bromide, and allyl iodide, to form an azeotropic mixture with the water in the aqueous acetic acid, distilling water and withdrawing agent from the mixture, condensing the distillate and substantially separating the withdrawing agent from the water therein, returning the separated withdrawing agent substantially continuously to the still, and supplying further aqueous acetic acid substantially continuously to the still, the withdrawing agent being returned to the still at a rate just right to form an azeotropic mixture with the water contained in the aqueous acetic acid being supplied to the still.

9. The process of concentrating aqueous acetic acid which comprises mixing therewith just the right amount of iso-amyl chloride to form an azeotropic mixture with the water in the aqueous acetic acid and distilling water and iso-amyl chloride from the mixture.

10. The process of concentrating aqueous acetic acid to produce a concentrated acid free of isoamyl chloride which consists in mixing therewith a plurality of parts by weight of iso-amyl chloride but in a quantity not in excess of the azeotropic ratio of parts by weight of water contained in the aqueous acetic acid and distilling water and isoamyl chloride from the mixture.

11. The continuous process of concentrating aqueous acetic acid to produce a concentrated acid free of iso-amyl chloride which comprises charging a still with a mixture of aqueous acetic acid and not more than the right amount of isoamyl chloride to form an azeotropic mixture with the water in the aqueous acetic. acid, distilling water and iso-amyl chloride from the mixture, condensing the distillate and substantially separating the iso-amyl chloride from the water therein, returning the separated iso-amyl chloride substantially continuously to the still and supplying further aqueous acetic acid substantially continuously to the still, the iso-amyl chloride being returned to the still at a rate to supply iso-amyl chloride in a quantity not in excess of the right amount to form an azeotropic mixture with the water contained in the aqueous acetic acid being supplied to the still.

12. The continuous process of concentrating aqueous acetic acid to produce a concentrated acid free of withdrawing agent which comprises charging a column still with a mixture of aqueous acetic acid and approximately the rightamount of iso-amyl'chlorideto form an azeotropic mixture with the water in the aqueous acetic acid, distilling water and iso-amyl chloride from the mixture, condensing the distillate and substantially separating the iso-amyl chloride from the water therein, returning the separated iso-amyl chloride 1 substantially continuously to the still and supplying further aqueous acetic acid substantially continuously to the still, the iso-amyl chloride being returned to the head of the column still at a rate not more than suflicient to form an azeo- 15 tropic mixture with the major proportion of the water contained in the aqueous acetic acid being supplied to the still and to the column still at a point somewhat below the head of the column at a rate approximately suflicient to form an azeotropic mixture with the remaining water contained in the aqueous acetic acid being supplied to the still.

13. The process of concentrating aqueous acetic acid which comprises mixing therewith just theright amount of butyl bromide to form an azeotropic mixture with the water in the aqueous acetic acid and distilling water and butyl bromide from the mixture.

14. The continuous process of concentrating aqueous acetic acid to produce a concentrated acid free of butyl bromide which comprises charging a still with a mixture of aqueous acetic acid and just the right amount of butyl bromide to form an azeotropic mixture with the water in 35 the aqueous acetic acid, distilling water and butyl bromide from the mixture, condensing the distillate and substantially separating the butyl bromide from the water therein, returning the separated butylbromide substantially continuously to (,0 the still and supplying further aqueous acetic acid substantially continuously to the still, the butyl bromide being returned to the still at a rate just right to form an azeotropic mixture with the water contained in the aqueous acetic acid being 45 supplied to the still.

15. The continuous process of concentrating aqueous acetic acid by azeotropic distillation to produce a concentrated acid free of withdrawing agent which comprises supplying a column still 50 with aqueous acetic acid and a water withdrawing agent selected from the group consisting of iso-propyl iodide, dichlorobrommethane, isobutyl bromide, dibrommethane, iso-amyl chloride, normal-butyl bromide, and allyl iodide, the quan- 55 tity of withdrawing agent supplied to the still, the rate of heating of the still and the rate of supplying aqueous acetic acid to the still being so controlled that there will be no excess of withdrawing agent over its azeotropic ratio with 60 the water contained in the still, and that at the point in the column at which substantially the last of the water distills off, the last of the withdrawing agent will also distill off.

16. Tile continuous process of concentrating 65 aqueous acetic acid to produce a concentrated acid free of withdrawing agent which comprises charging a still with a mixture of aqueous acetic acid and not more than the right amount of a waterwithdrawing agent, selected from the group con- 70 sisting of iso-propyl iodide, dichlorobrommethane, iso-butyl bromide, dibrommethane, iso-amyl chloride, normal-butyl bromide, and allyl iodide, to form an azeotropic mixture with the water in the aqueous acetic acid, distilling water and with- 15 drawing agent from the mixture, condensing the distillate and substantially separating the withdrawing agent from the water therein, returning the separated withdrawing agent substantially continuously to the still, supplying further aqueous acetic acid substantially continuously to the still, the withdrawing agent being returned to the still at a rate to supply a quantity of withdrawing agent not more than suflicient to form an azeotropimmixture with the water contained in the aqueous acetic acid being supplied to the still, and further distilling water and withdrawing agent from the mixture, thereby leaving a residue containing concentrated acetic acid free of withdrawing agent, heating said residue sufliciently to cause at least some of the concentrated acid to exist in the vapor phase, conducting away these acetic acid vapors free of withdrawing agent and condensing these vapors to the liquid phase.

DONALD F. OTHMER. 

